A transmitter for use in communication and broadcasting equipment, such as a portable telephone system and wireless Local Area Network (LAN) equipment, is required to operate with low power consumption while maintaining a transmission waveform at high accuracy independently of a level of transmission power. In recent years, a digital transmitter using a delta-sigma modulator and a class-D amplifier in combination has attracted attention as a transmitter that is expected to provide high power efficiency. As illustrated in FIG. 10, the digital transmitter includes a baseband signal generator, a delta-sigma modulator, a class-D amplifier, a band-pass filter, and an antenna.
An input signal generated by the baseband signal generator is delta-sigma modulated by the delta-sigma modulator and quantized into a binary pulse string. The binary pulse string thus generated is amplified by the class-D amplifier while maintaining a pattern of the pulse string. Further, by being passed through the band-pass filter, the input signal in an amplified state is reconstructed. The class-D amplifier, which occupies most of power in the digital transmitter, can obtain theoretically 100% power conversion efficiency unless there is power loss caused by a parasitic element. Thus, the transmitter is expected to have higher efficiency as a whole.
Modulation schemes of the delta-sigma modulator in the digital transmitter include envelope delta-sigma modulation, low-pass delta-sigma modulation, and band-pass delta-sigma modulation. Configuration examples of the respective modulation schemes are disclosed in PTL 1, NPL 1, and NPL 2.
FIG. 11 illustrates a configuration of an envelope delta-sigma modulator disclosed in PTL 1. In a digital baseband, an I-component and a Q-component of an input signal generated by a baseband signal generator (not illustrated in the figure) are generated. An amplitude-phase conversion unit converts the I-component and the Q-component into an amplitude component r and a phase component θ. A pulse phase signal generation unit generates, based on the phase component θ, a pulse phase signal at a radio frequency (RF). A delta-sigma modulation unit delta-sigma modulates the amplitude component r with the pulse phase signal as a clock and generates a pulse amplitude signal. Finally, by multiplying the pulse phase signal by the pulse amplitude signal, a pulse string of a pulse modulation signal is generated and output.
The envelope delta-sigma modulation scheme has an advantage that zero current switching is established in a digital amplifier. FIG. 12 is a diagram describing a principle of zero current switching. A class-D amplifier in FIG. 12, which corresponds to the class-D amplifier in FIG. 10, amplifies voltage of a pulse generated by a delta-sigma modulator and outputs the voltage. At this time, looking at an output voltage (VOUT) and an output current (IOUT), the output current always becomes 0 at switching points of the output voltage.
When a large output current is generated in a process of switching of an output voltage from High to Low and from Low to High, IV overlapping occurs in a switching element of a class-D amplifier, which may possibly cause power loss at the class-D amplifier. In a case of the envelope delta-sigma modulation scheme, since a phase of a pulse string always matches with a phase of a desired frequency included in the pulse string, an output current always becomes 0 at timings of voltage switch of the pulse string. As a result, no power loss is generated by IV overlapping. Thus, the class-D amplifier realizes further higher-efficiency amplification.